Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Like Xu <like.xu@linux.intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	wei.huang2@amd.com, Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	linux-kernel@vger.kernel.org, Li RongQing <lirongqing@baidu.com>,
	Chai Wen <chaiwen@baidu.com>, Jia Lina <jialina01@baidu.com>
Subject: Re: [PATCH] KVM: X86: Emulate APERF/MPERF to report actual VCPU frequency
Date: Tue, 23 Jun 2020 11:29:10 -0700	[thread overview]
Message-ID: <20200623182910.GA24107@linux.intel.com> (raw)
In-Reply-To: <20200623063530.81917-1-like.xu@linux.intel.com>

On Tue, Jun 23, 2020 at 02:35:30PM +0800, Like Xu wrote:
> The aperf/mperf are used to report current CPU frequency after 7d5905dc14a
> "x86 / CPU: Always show current CPU frequency in /proc/cpuinfo". But guest
> kernel always reports a fixed VCPU frequency in the /proc/cpuinfo, which
> may confuse users especially when turbo is enabled on the host.
> 
> Emulate guest APERF/MPERF capability based their values on the host.
> 
> Co-developed-by: Li RongQing <lirongqing@baidu.com>
> Signed-off-by: Li RongQing <lirongqing@baidu.com>
> Reviewed-by: Chai Wen <chaiwen@baidu.com>
> Reviewed-by: Jia Lina <jialina01@baidu.com>
> Signed-off-by: Like Xu <like.xu@linux.intel.com>
> ---

...

> @@ -8312,7 +8376,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		dm_request_for_irq_injection(vcpu) &&
>  		kvm_cpu_accept_dm_intr(vcpu);
>  	fastpath_t exit_fastpath;
> -
> +	u64 enter_mperf = 0, enter_aperf = 0, exit_mperf = 0, exit_aperf = 0;
>  	bool req_immediate_exit = false;
>  
>  	if (kvm_request_pending(vcpu)) {
> @@ -8516,8 +8580,17 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
>  	}
>  
> +	if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap))
> +		get_host_amperf(&enter_mperf, &enter_aperf);
> +
>  	exit_fastpath = kvm_x86_ops.run(vcpu);
>  
> +	if (unlikely(vcpu->arch.hwp.hw_coord_fb_cap)) {
> +		get_host_amperf(&exit_mperf, &exit_aperf);
> +		vcpu_update_amperf(vcpu, get_amperf_delta(enter_aperf, exit_aperf),
> +			get_amperf_delta(enter_mperf, exit_mperf));
> +	}
> +

Is there an alternative approach that doesn't require 4 RDMSRs on every VMX
round trip?  That's literally more expensive than VM-Enter + VM-Exit
combined.

E.g. what about adding KVM_X86_DISABLE_EXITS_APERF_MPERF and exposing the
MSRs for read when that capability is enabled?